The present invention relates to a technique suited to a mobile communication system in which data transmission is performed using multicarriers such as an orthogonal frequency and code division multiplexing (OFCDM) system and an orthogonal frequency division multiplexing (OFDM) system.
In the OFCDM system, code division multiplexing is performed using spreading codes in the orthogonal frequency division multiplexing (OFDM) system, where data is sent in parallel by using multiple sub carriers. The OFCDM system can achieve user multiplexing at the same frequency.
The OFCDM system has the following advantages.    (1) Immunity to narrowband interference    (2) Resistance to frequency selective fading    (3) High frequency use efficiency (because a frequency can be shared by sub carriers)    (4) Frequency domain processing can be performed.            An outline of an OFCDM transmission method is described below (see Patent Document 1, for example).            <1> First, a transmission target information symbol is prepared.    <2> Error-correction encoding such as turbo encoding is applied to the information symbol.    <3> Predetermined data modulation processing (for example, QPSK modulation) is applied to the information symbol, to which the error-correction encoding has been applied.    <4> Serial-parallel conversion is applied to the modulated information symbol.    <5> According to a predetermined spreading factor (SF), the same number of copies of the modulated information symbol, to which the serial-parallel conversion has been applied, are made as the number of spreading cycles (spreading factor) of spreading code.    <6> Each of the information symbols obtained by the copying is multiplied by one chip of spreading code.    <7> The information symbols, each of which has been multiplied by spreading code, are multiplexed.    <8> Frequency and time transformation (inverse fast Fourier transformation (IFFT)) processing is applied to the multiplexed information symbols. As a result, an OFCDM transmission signal (orthogonal multicarrier signal) is generated.    <9> A guard interval (GI) is inserted to each of the information symbols in the OFCDM transmission signal.    <10> The OFCDM transmission signal which includes spread parallel symbols, is sent by radio communication using carriers (multicarriers) of various frequencies.
In the OFCDM system, transmission is performed using multiple sub carriers as shown in FIG. 22. Accordingly, even when frequency selective fading occurs, the fading influences only a certain sub carrier. Further, in the influenced sub carrier, this influence can be regarded as flat fading (simple attenuation). In this case, a signal-to-interference ratio (SIR) of the sub carrier is lower than a target value. Therefore, when the transmission power of the sub carrier is increased through sub carrier transmission power control (STPC), a problem to be caused by the frequency selective fading can be prevented.
In contrast to this, in a case of a single carrier as used in the code division multiple access (CDMA) system, the occurrence of frequency selective fading influences the entire transmission carrier. Accordingly, the entire transmission data is influenced. It was difficult to reduce this influence through transmission power control (TPC). In other words, when the single carrier is used, it was difficult to improve a bit error rate by reducing the influence caused by the frequency selective fading.
As described above, in a system in which radio transmission is performed using the sub carriers such as the OFCDM system, an influence caused by the frequency selective fading can be more suppressed through the STPC than in radio transmission using the single carrier.
However, the STPC may influence another call. Specifically, as shown in FIG. 23, in a certain mobile station ((MS): mobile terminal) #0, when a reception power level is dropped by an influence caused by the frequency selective fading, the mobile station #0 requests to increase transmission power. As a result, the mobile station #0 can ensure a desired SIR. However, such an increase in the transmission power may lead to an increase in noise component of a corresponding sub carrier to be received by another mobile station #1.
In order to solve this problem, there is proposed a method in which a sub carrier at which a reception level has been dropped is not used (for example, see Non-Patent Document 1).
Further, in view of a problem of a reduced transmission efficiency caused when such a sub carrier is simply not used, there is proposed a partial non-power allocation (PNPA) system in which punctured bits are allocated to a sub carrier that is not used (for example, Non-Patent Document 2).
In a diversity hand over (DHO) system in which a mobile station can communicate with multiple base stations, in 3GPP systems, identical data is sent to all transmission paths (branches) by a single carrier. In the reception side, data reproduction is performed by combining the pieces of data sent through the branches. In this case, there is a problem in that, when each piece of data is influenced by frequency selective fading in each of the branches, even if the reception side selects any combination of the data received from the branches when combining the data, reproduced data is also influenced by the frequency selective fading.
When soft hand over is performed, identical data flows through multiple transmission paths. This implies inefficiency in the use of channels while ensuring quality. For example, when a radio network controller ((RNC): base station control device) includes a function unit which performs data selection and combining, identical data is sent through each interface “Iub” (interface between the RNC and a node B (base station)), for both an uplink and a downlink.
Further, for uplink transmission, the mobile station sends identical data to all base stations, in 3GPP systems.                Patent Document 1: JP 2004-134978 A        Non-Patent Document 1: Toshimitsu TSUBAKI, Yoichi MATSUMOTO, Masahiro UMEHIRA, “Study of ARQ for wireless ATM using OFDM sub carrier information”, Proceedings 1 of The Institute of Electronics, Information and Communication Engineers (IEICE) communication society conference in 1997, IEICE, p. 332, Aug. 13, 1997.        Non-Patent Document 2: Noriyuki MAEDA, Seiichi SAMPEI, Norihiko MORINAGA, “Characteristics in a sub carrier transmission power control method based on a delay profile information channel, for OFDM/FDD systems”, IEICE Transactions on communications, Vol. J84-B No. 2, IEICE, pp. 205-207, 398th volume, Feb. 1, 2001.        